Acrolein has been industrially widely used as a starting material for acrylic acid and the like, and acrylic acid, as starting material for water-absorbent resin. Generally practiced production processes of acrolein or acrylic acid comprise catalytic gas-phase oxidation of propylene, or catalytic gas-phase oxidation of acrolein, using a fixed bed shell-and-tube reactor, in the presence of an oxidation catalyst.
This catalytic gas-phase oxidation reaction is a highly exothermic reaction, and hence locally extraordinarily high temperature spots (hereafter may be referred to as “hot spots”) are formed in the catalyst layers. At the hot spots the oxidation reaction progresses excessively, because of the higher temperature than that at the rest of the reaction area, to reduce yield of the object product, i.e., acrolein or acrylic acid. Moreover, because the hot spots are exposed to high temperatures, the catalyst at the spots shows changes in physical and chemical properties within a short period, which leads to notable decrease in its activity or selectivity due to sintering or the like. In particular, when the catalyst contains molybdenum, high temperatures at hot spots accelerate sublimation of the molybdenum to cause changes in the catalyst's composition (ratios among its constituents) and therefore the deterioration extent of the catalyst is large. There is still another problem that this phenomenon is enhanced when the reaction is carried out at high space velocity or high starting gas concentration, i.e., when a high-load reaction is carried out, for improving productivity of acrolein or acrylic acid.
In order to cope with these problems, various improvement means have been proposed for preparing acrolein from propylene, for example, a method comprising filling the reaction tube with plural kinds of catalysts differing in occupation volume, mixing in at least one of the reaction zones an inert substance molding (Patent Reference 1: JP 2005-320315A=U.S. Pat. No. 7,161,044); a method using a supported catalyst, comprising filling the reaction tube with plural kinds of catalysts having different activities which are prepared by varying the supported ratio of catalytically active component on the catalysts and/or calcining temperature, in such a manner that the activity becomes higher from the starting gas-inlet side toward the outlet side (Patent Reference 2: JP Hei 10 (1998)-168003A=U.S. Pat. No. 6,028,200); or a method of filling the reaction tube with plural catalysts in such a manner that the occupation volume of catalyst filled in the reaction tube becomes lower from the starting gas-inlet side toward the outlet side (Patent Reference 3: JP Hei 4 (1992)-217932A=U.S. Pat. No. 5,198,581).
Concerning preparation of acrylic acid from acrolein, various improvements have also been proposed, for example, a method comprising diluting the catalyst at the starting gas-inlet side with an inert substance (Patent Reference 4: JP Sho 53 (1978)-30688B=U.S. Pat. No. 3,801,634); a method using a supported catalyst, comprising filling the reaction tube with the catalyst in which supported ratio of catalytically active component becomes higher from the starting gas-inlet side toward the outlet side (Patent Reference 5: JP Hei 7 (1995)-10802A); a method comprising preparing plural catalysts having different activities by varying the kind and/or amount of alkali metal added to the catalyst, and filling them in the reaction tube in such a manner that the catalytic activity becomes higher from the starting gas-inlet side toward the outlet side (Patent Reference 6: JP 2000-336060A=U.S. Pat. No. 6,563,000); and a method comprising filling the reaction tube in such a manner that the volume of catalyst particles become smaller from the starting gas-inlet side toward the outlet side (Patent Reference 7: JP Hei 9 (1997)-241209A=U.S. Pat. No. 5,719,318).
However, such means as dilution with inactive substance, varying the supported ratio or use of catalysts having different activities reduce the amount of the catalytic component effective for the reaction, which is filled in the reaction tube, to impair productivity of acrylic acid. Preparation of plural catalysts also requires much labor and huge cost, compared to preparation of single kind of catalyst.
Still in addition, while all of above proposals accomplish improvement in suppressing temperature at hot spots to a certain extent, the effect is yet unsatisfactory. That is, it is the current situation that there is still room for improvements in those methods in respect of both catalyst life and yield of acrolein or acrylic acid.